Under barbiturate anesthesia, microelectrode recordings from the trigeminal ganglion of monkey are performed and electrical activity is recorded and stored on magnetic tape. Thermal and noxious heat stimuli are delivered with a precisely-controlled thermode which has a temperature range of 20 degrees C to 60 degrees C and can produce a temperature change of 10 degrees C at the thermode-skin junction within one second. Data are analyzed with the aid of an on-line real-time computer system. In related experiments, intracellular recordings are made from primary afferent neurons in trigeminal dorsal root ganglia, dorsal root fibers, or the spinal trigeminal tract, in response to innocuous and noxious thermal and mechanical stimuli. After functional characterization, horseradish peroxidase is iontophoresed into the cell or fiber. This intracellular label is then located in histological material and the central terminal projections of each axon determined.